I would like to model fluid flow with additional EXPLICIT SOURCE TERM appearing in a momentum equation - here I would like to be able to define this source term as a function of time ("switch it on and of") as well as geometry (small region inside the fluid domain).Can this can be done by timeActivatedExplicitSource.C? If so, can anyone explain me "how and what"!

You'll have to add it to the the relevant transport equation. Just add a field and you're done. The question is how you get the values into the field. For that there are several optinos:
- Programing by hand
- Using the scalarTimeActivatedExplicitSourceList that comes with OF (I guess. Havn't worked with that)
- Swak4Foam has the possibility to have arbitrary source terms based on an expression. For a discussion see http://www.cfd-online.com/Forums/ope...ak4foam-4.html